Low-noise oscillator amplitude regulator

ABSTRACT

A frequency generation solution controls an oscillator amplitude using two feedback paths to generate high frequency signals with lower power consumption and lower noise. A first feedback path provides continuous control of the oscillator amplitude responsive to an amplitude detected at the oscillator output. A second feedback path provides discrete control of the amplitude regulating parameter(s) of the oscillator responsive to the detected oscillator amplitude. Because the second feedback path enables the adjustment of the amplitude regulating parameter(s), the second feedback path enables an amplifier in the first feedback path to operate at a reduced gain, and thus also at a reduced power and a reduced noise, without jeopardizing the performance of the oscillator.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/577,973, which was filed on Nov. 29, 2017, which is anational stage application of PCT/EP2016/060873, filed May 13, 2016,which is a continuation of U.S. patent application Ser. No. 14/731,487,filed Jun. 5, 2015, which issued as U.S. Pat. No. 9,473,151 on Oct. 18,2016, the disclosures of which are incorporated herein by reference intheir entirety.

TECHNICAL FIELD

The solution presented herein relates generally to frequency generation,and more particularly to reducing phase noise and power consumption ofhigh frequency generation circuits.

BACKGROUND

Oscillators are widely used in various electronic devices, e.g., toprovide reference clocks, mixing frequencies for telecommunicationsignals, etc. A negative resistance-based oscillator represents one typeof oscillator architecture typically used for the generation of higherfrequency signals, such as used in wireless communication devices.Examples of negative resistance-based oscillators include, but are notlimited to crystal oscillators, Surface Acoustic Wave (SAW)-basedoscillators, etc. Negative resistance-based oscillators comprise anoscillator core having a resonant circuit operatively connected to anegative resistance circuit. The resonant circuit oscillates at thedesired resonant frequency, and the negative resistance circuit cancelsthe resistive losses of the resonant circuit. In effect, the negativeresistance circuit eliminates the natural damping of the resonantcircuit, and therefore enables the oscillator core to continuouslyoscillate at the desired resonant frequency.

The successful operation of electronic devices containing suchoscillators requires accurate and reliable amplitude control. Inparticular, amplitude control is necessary due to the fact thatdifferent Q-values, e.g., of different resonant circuits, as well asdifferent PVT (Process, Voltage, and Temperature) conditions for any oneoscillator may cause wide amplitude variations. For example, anoscillator having a high-Q resonant circuit will have higher amplitudeoscillation than an oscillator having a low-Q resonant circuit. Further,an oscillator running in a linear mode requires continuous regulation ofthe amplitude to prevent the oscillator amplitude from quickly fallingto zero or increasing to a level limited by the non-linear effects,e.g., voltage clipping, of the oscillator. Such voltage clipping cangreatly deteriorate oscillator performance, increase the risk ofparasitic oscillation, increase the current consumption (depending oncircuit topology), and generally make the behavior of the oscillatormore unpredictable. Accurate and reliable amplitude control willequalize the amplitude variations across a wide range of Q-values andPVT conditions, as well as ensure good noise performance, provide lowcurrent consumption, avoid parasitic oscillation, and possibly preventdamage to active and passive components

A negative feedback loop provides one way to control the amplitude ofthe oscillator output, where the negative feedback loop senses theamplitude of the oscillator output and then adjusts the amplitude bycontrolling an operating point of the oscillator core. For example,controlling the current through active transistor devices of theoscillator core controls the transconductance g_(m) of the oscillatorcore to control the negative resistance, and thus controls theoscillator amplitude. However, such negative feedback loops mayintroduce noise into the oscillator core, particularly when the negativefeedback loop has a high gain. Further, the nonlinear properties of theoscillator core will convert the input noise to both AM (AmplitudeModulation) and PM (Phase Modulation) noise. While increasing the loopgain of the negative feedback loop will reduce the AM noise, such anincreased loop gain will not only increase the power consumption, butwill also fail to reduce the PM noise. While reducing the bandwidth ofthe negative feedback loop will also reduce the noise, such a bandwidthreduction, however, will increase the startup time of the oscillator,and may also undesirably increase the size (consumed chip area) of anyfilter required to filter the oscillator input signal. Thus, suchbandwidth reduction is also not desirable.

As noted above, negative resistance-based oscillators are particularlyuseful for high frequency applications, and may be particularlyimportant for mmW (millimeter wave) communication. Also, specificallyfor reference oscillators based on e.g., crystal or SAW resonators, theuse of even higher frequencies is anticipated, from todays 10's of MHzto 100's of MHz and possibly even frequencies approaching the GHz range.The generation of such higher frequencies generally results in higherpower consumption. Further, the generation of such higher frequenciesalso presents design challenges due to increased tolerances of theresonators, increased noise, increased component sizes, longer startuptimes, and/or larger impacts from parasitic elements of the circuitryand associated package. Thus, there remains a need for improved higherfrequency generation circuits that do not incur higher powerconsumption, higher noise, and/or longer start-up times.

SUMMARY

The solution presented herein generates high frequency signals withlower power consumption and lower noise by controlling an oscillatoramplitude using two feedback paths. A first feedback path providescontinuous control of the oscillator amplitude responsive to anamplitude detected at the oscillator output. A second feedback pathprovides discrete control of the amplitude regulating parameter(s) ofthe oscillator responsive to the detected oscillator amplitude. Becausethe second feedback path enables the adjustment of the amplituderegulating parameter(s), the second feedback path enables an amplifierin the first feedback path to operate at a reduced gain, and thus alsoat a reduced power and a reduced noise, without jeopardizing theperformance of the oscillator.

One exemplary embodiment comprises a frequency generation circuitcomprising an oscillator, a detector, a first feedback path, and asecond feedback path. The oscillator comprises an oscillator output, afirst control input, and a second control input. The detector isconfigured to detect an amplitude of the oscillator output. The firstfeedback path operatively connects the detector to the first controlinput, and is configured to provide time-continuous control, responsiveto the detected amplitude, of the amplitude of the oscillator output bycontinuously controlling a first control signal applied to the firstcontrol input. The second feedback path operatively connects thedetector to the second control input, and is configured to providetime-discrete control, responsive to the detected amplitude, of one ormore amplitude regulating parameters of the oscillator by providingtime-discrete control of a second control signal applied to the secondcontrol input.

Another exemplary embodiment comprises a method of controlling anoscillator comprising an oscillator output, a first control input, and asecond control input. The method comprises detecting an amplitude of theoscillator output, and providing time-continuous control, responsive tothe detected amplitude, of the amplitude of the oscillator output bycontinuously controlling a first control signal applied to the firstcontrol input. The method further comprises providing time-discretecontrol, responsive to the detected amplitude, of one or more amplituderegulating parameters of the oscillator by providing time-discretecontrol of a second control signal applied to the second control input.

Another exemplary embodiment comprises a computer program product storedin a non-transitory computer readable medium for controlling anoscillator of a frequency generation circuit. The oscillator comprisesan oscillator output, a first control input, and a second control input.The computer program product comprises software instructions which, whenrun on the frequency generation circuit, causes the frequency generationcircuit to detect an amplitude of the oscillator output, and providetime-continuous control, responsive to the detected amplitude, of theamplitude of the oscillator output by continuously controlling a firstcontrol signal applied to the first control input. The softwareinstructions, when run on the frequency generation circuit, furthercause the frequency generation circuit to provide time-discrete control,responsive to the detected amplitude, of one or more amplituderegulating parameters of the oscillator by providing time-discretecontrol of a second control signal applied to the second control input.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a frequency generation circuit accordingto one exemplary embodiment.

FIG. 2 shows an amplitude control method according to one exemplaryembodiment.

FIG. 3 shows a block diagram of the first feedback path of the frequencygeneration circuit of FIG. 1 according to one exemplary embodiment.

FIG. 4 shows a block diagram of the second feedback path of thefrequency generation circuit of FIG. 1 according to one exemplaryembodiment.

FIG. 5 shows another amplitude control method according to one exemplaryembodiment.

FIG. 6 shows simulation results achievable with only a first feedbackpath having a high gain.

FIG. 7 shows simulation results achievable with only a first feedbackpath having a low gain.

FIG. 8 shows exemplary simulation results achievable with the solutionpresented herein.

FIG. 9 shows exemplary simulation results when the first feedback pathhas different gains.

FIG. 10 shows exemplary simulation results of the noise improvementachievable with the solution presented herein.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a frequency generation circuit 100according to one exemplary embodiment. For simplicity, FIG. 1 only showsthe elements of the frequency generation circuit 100 necessary tofacilitate the description provided herein. It will be appreciated bythose skilled in the art that the frequency generation circuit 100 mayinclude additional components and/or signal connections not shown inFIG. 1.

Frequency generation circuit 100 includes an oscillator 110 coupled tocontrol circuitry 115 that controls the amplitude of the oscillatoroutput. Oscillator 110 includes a first control input (CTRL₁), a secondcontrol input (CTRL₂), and an output (OUT). The oscillator 110 maycomprise a crystal oscillator, or any other negative resistance-basedoscillator that includes a resonant circuit 112 operatively connected toa negative resistance circuit 114. In one exemplary embodiment, theresonant circuit 112 may comprise a crystal, and the negative resistancecircuit 114 may comprise an amplifier (not shown). First and secondcontrol signals, S₁ and S₂, applied to the respective first and secondcontrol inputs control the amplitude of the signal S_(o) at the outputof the oscillator 110. In particular, the first control signal S₁provides time-continuous control of the amplitude of S_(o), while thesecond control signal S₂ provides time-discrete control of one or moreamplitude regulating parameters of the oscillator 110, as describedfurther below. Exemplary amplitude regulating parameters include, butare not limited to, an oscillator bias current, a number of activeoscillator g_(m) cells, a bias point of one or more of the oscillatorg_(m) cells, and/or a variable resistance connected in parallel with acore of the oscillator 110. Because the second control signal S₂controls the configuration of the oscillator 110, S₂ enables therelaxation of the requirements that would otherwise be placed on thetime-continuous amplitude control provided by the first control signalS₁.

The control circuitry 115 generates the first and second control signalsS₁, S₂ responsive to the oscillator output signal S_(o) according to theexemplary method 200 of FIG. 2. More particularly, the control circuitry115 comprises a detector 120, a first feedback path 130, and a secondfeedback path 140. The detector 120, which is coupled between theoscillator output and the inputs of the first feedback path 130 and thesecond feedback path 140, detects an amplitude A of the oscillatoroutput signal S_(o) (block 210). The first feedback path 130 providestime-continuous control of the amplitude of the oscillator output signalS_(o) by continuously controlling the first control signal S₁ responsiveto the detected amplitude A (block 220). The second feedback path 140provides time-discrete control of one or more amplitude regulatingparameters of the oscillator 110 by controlling, in discrete time, thesecond control signal S₂ responsive to the detected amplitude A (block230). For example, the second control signal may provide time-discretecontrol of the parameter(s) controlling the operation of the negativeresistance circuit 114. By controlling the amplitude regulatingparameter(s) of the oscillator 110, the second feedback path 140 allowsthe first feedback path 130 to operate at a lower gain, and therefore ata lower power and with less noise.

FIG. 3 shows a block diagram of the first feedback path 130 according toone exemplary embodiment. In this embodiment, the first feedback path130 includes an amplifier 132 and a filter 134. The detected amplitudeA, as well as a reference amplitude A_(ref), are input to amplifier 132.Amplifier 132 amplifies the amplitude error A_(err) formed from thedifference between the detected amplitude A and the reference amplitudeA_(ref), and filter 134 helps reduce the noise input to the oscillator110 by low-pass filtering the amplified signal to generate the firstcontrol signal S₁. The first control signal S₁ controls the gain of theoscillator core by controlling the gain of the negative resistancecircuit 114. In so doing, the first control signal S₁ controls theamplitude of the oscillator output signal S_(o).

Amplifier 132 establishes the gain of the first feedback path 130.Because various environmental conditions, oscillator properties, and/orthe age of the oscillator 110, may impact the ability of the firstcontrol signal S₁ to sufficiently control the amplitude of theoscillator output signal S_(o), conventional systems tend to set thegain of amplifier 132 to account for a wide range of conditions, even ifsome of the more extreme conditions are very rare. For example, highertemperatures may reduce the gain of the oscillator core relative to whatthat gain would be with the same input control signal at regularoperating temperatures. Conventional solutions address this problem bymaking sure the gain of amplifier 132 is high enough to enable theoscillator core to handle even extreme temperature conditions withoutdropping the amplitude of the oscillator output S_(o) below a desiredlevel. Such high gain conditions, however, cause amplifier 132 toconsume more power and to insert more noise into the oscillator corethan would otherwise be necessary for many operating conditions.

The solution presented herein incorporates the second feedback path 140into the control circuitry 115 to control the amplitude regulatingparameter(s) of the oscillator 110, which allows the first feedback path130 to be designed and configured for a lower gain. Such gain reductionin the first feedback path 130 will enable the frequency generationcircuit 100 to operate at a lower power and will reduce the noise levelinput to oscillator 110. To that end, the second feedback path 140controls one or more amplitude regulating parameters responsive to thedetected amplitude A of the oscillator output signal S_(o). For example,if the detected amplitude A drops too low, indicating that the firstcontrol signal is unable to sufficiently amplify the oscillatoramplitude, the second feedback path 140 may adjust the amplituderegulating parameters, e.g., by increasing the bias current, increasingthe number of active oscillator gm cells, and/or increasing a bias pointof one or more of the active gm cells. Alternatively or additionally,the second feedback path 140 may adjust the amplitude regulatingparameters by increasing the resistance of a variable resistanceconnected in parallel with the oscillator core, e.g., using a variableresistor 116 connected across differential outputs of the oscillator110. In another example, if the detected amplitude A rises too high,indicating the amplitude of the oscillator output signal S_(o) is toohigh, the second feedback path 140 may decrease the bias current,decrease the number of active oscillator gm cells, decrease a bias pointof one or more of the active gm cells, and/or decrease the resistance ofthe variable resistor 116 connected in parallel with the core of theoscillator 110. In either case, the second feedback path 140 adjusts theamplitude regulating parameter(s) for the current operating conditionsas indicated by the detected amplitude A to enable the oscillator 110 tomaintain the desired amplitude at the output without requiring the firstfeedback path 130 to have a high gain.

Because the gain of amplifier 132 is designed to handle most operatingconditions, the control provided by the second feedback path 140 may beimplemented in a time-discrete manner. For example, the second feedbackpath 140 may include a control circuit 142, as shown in FIG. 4. Controlcircuit 142 may control the amplitude regulating parameter(s) of theoscillator in a time-discrete manner by only controlling the amplituderegulating parameter(s) when the detected amplitude A satisfies one ormore predetermined conditions, e.g., threshold conditions. For example,the control circuit 142 may control the second control signal S₂ tocontrol the amplitude regulating parameter(s) only when the detectedamplitude A exceeds an upper threshold T_(U) or is lower than a lowerthreshold T_(L). In addition, the control circuit 142 may control thesecond control signal S₂ to control the amplitude regulatingparameter(s) only under certain operating conditions and/or responsiveto an event trigger. For example, control circuit 142 may control thesecond control signal S₂ to allow the amplitude regulating parameter(s)to change when the oscillator 110 powers on and/or when the oscillator110 is acting in response to some communication event trigger. However,because changing the amplitude regulating parameters during, e.g.,active communications, could disrupt the phase and/or frequency of theoscillator 110, the control circuit 142 may control the second controlsignal S₂ to prevent the amplitude regulating parameter(s) from changingduring such periods to prevent this disruption. The control circuit 142may therefore use, in addition to the threshold conditions, power on/offevents and/or communication event triggers to provide additionaltime-discrete control of the oscillator's amplitude regulatingparameter(s).

The exemplary method 250 of FIG. 5 provides a more detailed approach forcontrolling the oscillator 110 at startup. In this exemplary method 250,the oscillator 110 is powered on (block 202), and the process waitsuntil the oscillator 110 stabilizes (block 204). Once the oscillator 110stabilizes (block 204), the detector 120 detects the amplitude A of theoscillator output signal S_(o) (block 210). If the detected amplitude Aexceeds an upper threshold T_(U) (block 232) or is less than a lowerthreshold T_(L) (block 234), the control circuit 142 in the secondfeedback path 140 determines the oscillator 110 is unable to maintain adesired amplitude with the current configuration. In response, thecontrol circuit 142 therefore alters one or more amplitude regulatingparameters of the oscillator 110 (block 236). Blocks 210, 232, and 234may be repeated once the oscillator 110 stabilizes again (block 204).This repetition may be indefinite, or may terminate after somepredetermined maximum number of iterations.

FIGS. 6-10 show simulation results to demonstrate the advantages of thesolution presented herein. FIGS. 6 and 7 first show the oscillationamplitude achievable when the control circuitry 115 does not include thesecond feedback path 140. In this case, the amplitude regulatingparameters of the oscillator 110 are fixed and the first feedback path130 provides the only amplitude control. FIG. 6 provides results whenamplifier 132 in the first feedback path 130 is configured to operatewith a high gain that results in a relatively high loop gain, e.g.,greater than 10, versus the results in FIG. 7 where the amplifier 132operates with a lower gain that results in a relatively low loop gain,e.g., less than 5. As shown by FIG. 6, the higher loop gainimplementation provides a very low amplitude variation, e.g., 50-55% ofthe full swing. However, the high gain necessary to achieve this lowamplitude variation results in high power consumption and high noiselevels. The lower loop gain implementation enables lower powerconsumption and noise levels, but as shown in FIG. 7, this lower loopgain implementation has a relatively high amplitude variation, e.g.,48-68% of the full swing.

FIG. 8 shows the results when the second feedback path 140 is includedwith the control circuitry 115 to enable time-discrete adjustment of theamplitude regulating parameter(s) of the oscillator 110. In thissimulation, the first feedback path 130 has a low gain and the secondfeedback path 140 is used to control two extra amplitude regulatingparameters, e.g., the bias tail current and/or the number of g_(m) cellsin the oscillator core, as shown by the three curves in FIG. 8. As shownby FIG. 8, the solution presented herein results in a lower amplitudevariation (52-60%), which was previously not achievable when the firstfeedback path 130 had a lower loop gain. Thus, the solution presentedherein provides the lower noise and power consumption benefits moretypically associated with lower loop gain implementations while alsoproviding the amplitude control benefits more typically associated withhigher loop gain implementations.

FIG. 9 shows simulation results demonstrating how the gain of amplifier132 may be selected to achieve the desired trade-off between amplitudecontrol and noise/power reduction. The results in FIG. 9 demonstrate theoscillator amplitude performance for six scenarios, which arequalitatively specified at each point, e.g., “high loop gain,” “low loopgain including second feedback path,” etc. The first four scenarios showthe amplitude performance for high/low loop gain and high/low Qscenarios when the second feedback path 140 is not included. The lasttwo scenarios show the amplitude performance for low loop gain andhigh/low Q scenarios when the second feedback path 140 is included.

FIG. 10 shows simulation results demonstrating the noise performance forthe same six scenarios as in FIG. 9, and thus demonstrates the noiseimprovement provided by the solution presented herein. In particular,the top two plots show the operation of the frequency generation circuit100 when the amplitude regulating parameters are fixed and the loop gainof the first feedback path 130 is high. The bottom plot shows theresults when the second feedback path 140 is used to modify the biascurrent and the g_(m) cells of the oscillator core when the loop gain ofthe first feedback path 130 is low. The solution presented hereintherefore provides a frequency generation circuit having the amplitudecontrol benefits associated with high gain negative feedback and thepower and noise benefits associated with low gain negative feedback.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A frequency generation circuit comprising: anoscillator comprising an oscillator output, a first control input, and asecond control input; a detector configured to detect an amplitude ofthe oscillator output; a first feedback path operatively connecting thedetector to the first control input, the first feedback path configuredto control the amplitude of the oscillator output responsive to thedetected amplitude by continuously applying a first control signal tothe first control input; and a second feedback path operativelyconnecting the detector to the second control input, the second feedbackpath configured to control one or more amplitude regulating parametersof the oscillator responsive to the detected amplitude by applying asecond control signal to the second control input; wherein the secondfeedback path comprises a control circuit; and wherein the controlcircuit is configured to prevent changes to the one or more amplituderegulating parameters when the oscillator has established frequencysynchronization with one or more external devices.
 2. The frequencygeneration circuit of claim 1 wherein the first feedback path isconfigured to control the amplitude of the oscillator output, responsiveto the detected amplitude, by continuously controlling a gain of theoscillator.
 3. The frequency generation circuit of claim 1 wherein thecontrol circuit is configured to control the one or more amplituderegulating parameters responsive to the detected amplitude.
 4. Thefrequency generation circuit of claim 3 wherein the control circuitcomprises a first comparison circuit configured to compare the detectedamplitude to an upper threshold, and wherein the control circuit isconfigured to control the one or more amplitude regulating parameters bycontrolling the one or more amplitude regulating parameters when thedetected amplitude exceeds the upper threshold.
 5. The frequencygeneration circuit of claim 3 wherein the control circuit comprises asecond comparison circuit configured to compare the detected amplitudeto a lower threshold, and wherein the control circuit is configured tocontrol the one or more amplitude regulating parameters by controllingthe one or more amplitude regulating parameters when the detectedamplitude is less than the lower threshold.
 6. The frequency generationcircuit of claim 1, wherein the one or more amplitude regulatingparameters comprise at least one of: an oscillator bias current; anumber of oscillator g_(m) cells; a bias point of one or more of theoscillator g_(m) cells; and a variable resistance connected in parallelwith a core of the oscillator.
 7. A method of controlling an oscillatorcomprising an oscillator output, a first control input, and a secondcontrol input, the method comprising: detecting an amplitude of theoscillator output; controlling the amplitude of the oscillator output,responsive to the detected amplitude, by continuously applying a firstcontrol signal to the first control input; and controlling one or moreamplitude regulating parameters of the oscillator responsive to thedetected amplitude by applying a second control signal to the secondcontrol input; and preventing changes to the one or more amplituderegulating parameters when the oscillator has established frequencysynchronization with one or more external devices.
 8. The method ofclaim 7 wherein controlling the amplitude of the oscillator outputsignal comprises continuously controlling a gain of the oscillatorresponsive to the detected amplitude.
 9. The method of claim 8 furthercomprising comparing the detected amplitude with an upper threshold,wherein controlling the one or more amplitude regulating parameterscomprises controlling the one or more amplitude regulating parameterswhen the detected amplitude exceeds the upper threshold.
 10. The methodof claim 8 further comprising comparing the detected amplitude with alower threshold, wherein controlling the one or more amplituderegulating parameters comprises controlling the one or more amplituderegulating parameters when the detected amplitude is less than the lowerthreshold.
 11. The method of claim 7 wherein the one or more amplituderegulating parameters comprise at least one of: an oscillator biascurrent; a number of oscillator gm cells; a bias point of one or more ofthe oscillator gm cells; and a variable resistance connected in parallelwith a core of the oscillator.
 12. A wireless communication devicecomprising a frequency generation circuit, said frequency generationcircuit comprising: an oscillator comprising an oscillator output, afirst control input, and a second control input; a detector configuredto detect an amplitude of the oscillator output; a first feedback pathoperatively connecting the detector to the first control input, thefirst feedback path configured to control the amplitude of theoscillator output responsive to the detected amplitude by applying afirst control signal to the first control input; and a second feedbackpath operatively connecting the detector to the second control input,the second feedback path configured to control one or more amplituderegulating parameters of the oscillator responsive to the detectedamplitude by applying a second control signal to the second controlinput; wherein the second feedback path comprises a control circuit; andwherein the control circuit is configured to prevent changes to the oneor more amplitude regulating parameters when the oscillator is beingused for wireless communications.
 13. The wireless communication deviceof claim 12 wherein the control circuit provides the control of the oneor more amplitude regulating parameters of the oscillator responsive toan event trigger, and wherein the event trigger comprises acommunication event trigger indicating an upcoming radio communication.14. The wireless communication device of claim 13 wherein thecommunication event comprises an upcoming random access channeltransmission event, an upcoming radio transmission event, or an upcomingradio reception event.